Gyratory impact ball mill and grinding method



March 14, 1950 L. G. SYMONS GYRATORY IMPACT BALL MILL AND GRINDING METHOD 4 Sheets-Sheet 1 Filed Dec. 26, 1947 March 14, 1950 L. s. SYMONS GYRATORY IMPACT BALL MILL AND GRINDING METHOD Filed Dec. 26, 1947 4 Sheets-Sheet 2 J???" 2 Bl r March 14, 1950 G. SYMONS GYRATORY IMPACT BALL MILL AND GRINDING METHOD 4 Sheets-Sheet 3 Filed Dec. 26, 1947 March 14, 1950' s. SYMONS GYRATORY IMPACT BALL MILL AND GRINDING METHOD 4 Sheets-Sheet 4 Filed Dec.

WWW @W rnemd Moi-r14, 1950 GYRATORY IMPACT BALL MILL AND- GRINDING METHOD Loren G. Symons, Hollywood, Calif., auignor to Nordberg Manufacturing Company, Milwaukee,

Wis a corporation of Wisconsin Application December 28, 1947, Serial No. 794,053

18' Claims.

My invention relates" to an improvement-in fine reduction or fine grinding-of materials, such as rock and ore. One purpose is to provide an improved method for reducing relatively small particles to substantially finer size.

Another purpose is to provide an improved of milling or grinding fine-particles.

Another purpose is to provide a method of grinding fine particles in which a mass of particles undergoing grinding is subjected to violent impact with a minimum of'abrasion orattrition.

Another purpose is to provide a method of grinding in which reduction and classification take place substantially simultaneously.

Another purpose is to provide a grinding method of improved efiicie'ncy and economy.

Another purpose is to prevent wear or grinding or abrasion of the wearing parts employed in connection with the practice of my grinding method.

Another purpose is to providean improved fine Brinding machine.

Another purpose is to provides. fine grinding machine or ball mill with an adjustable discharge aperture.

Another purpose is to provide an improved ball mill of maximum simplicity and compactness.

Another purpose is to provide improved weartaking parts for a ball mill.

Another purpose is to provide improvedmeans for feeding material to abail mill.

Other purposes will appear from time-to time in the course of the specification and claims.

The present application is a continuation-inpart of my application Serial No. 540,629, filed on June 16, 1944, now abandoned, for Method of fine grindingby impact of massive support and loose grinding elements." 7 The method-herein claimed may be practiced by the structure 'of nay; Patent No. 2,433,3'l2itshed on January 6, 1 8.

I illustrate the invention more orless diagrammatically in the accompanying drawings wherein:

Figure l is a vertical axial section; 4

Figure 2 is a section'on ithelhie- 2-2 ure 1-, on aslightly reduced scale; I I

Figure 3 is a section on the "line 3'3 of Figure 2, on an enlarged-scale;

of 1 Fig- Figure 4 isaplanview cfa" variant form of device: and

E'lgure'B is a section u a- Like parts are indicated by like symbols toushou th s ec fi a on a d .d awlnesbathe-line ts ci 2 Referring to the drawings, I generallyindicates any suitable base or support, herein shown as of concrete. Positioned upon it is a machine base 2 having a generally central downward extension 3, terminating in a cylindrical sleeve 4. The base structure includes an upwardly extending outer or circumferential wall 5 and aninner sleeve 8, terminating in a ring i having a downwardly tapered inner seat 8 which receives the normally, fixed central shaft or pedestal 9. The pedestal 9 has a reduced lower end portion it, terminating in a cylindrical end portion H' which is received in the sleeve 4. The'portion II. has :a further extension I! screw-threaded as at it to receive a locking nut N which exerts a thrust upwardly against the lower end of the sleeve 4 and draws a tapered intermediate portion 9a of the shaft or pedestal 5 against the tapered surface or seat 8 within the ring 7. Thus-the shaft or pedestal 9 is firmly fixed in position in relation to the base 2, without the necessity of any bearing or support for that part of it which extends upwardly above the base.

Rotatable about the shaft 9 is an eccentric 20. 2! is any suitable bearing sleeve of bronze-or other suitable material. The eccentric is rotatably supported upon any suitable roller thrust bearings 22, adapted tocarry the weight ofthe eccentric 20 and the slight amount of downthrust, due to the angularity of the eccentric. The eccentric is rotated by any suitable countershaft 23, shown as having a drive pulley .243 and a bevel pinion 25 in mesh with a gear 28 suitably secured to the bottom of the eccentric Z9.

' 2? is any suitable counterweight for the eccentric. 28 is any suitable sleeve or housing for the countershaft 28, and carries at its ends suitable roller bearings 28 and 30 in which the shaft 23 is ro tated.

Positioned at the top of the shaft or lm't-t is a ball or hemisphere .31, which may, for example, be of bronze. The bronze ball 3| and the conforming socket .32, which may be of steel, carry the weight of the below described bowl structure which surrounds and is gyrated by the eccentric 29. Bevel gears 33, 34 are mounted on the lower part of the ball member 3| and on the central sleeve .35 of the bowl structure. The gears are identical, and have the same number of teeth, and are adaptedto keep the bowl structure from turning around the fixed central'post s. Thus the bowl structure, as a whole, is permitted osyrate about a point'X, but is not permitted to rotate. .The ball' may be mounted in any suit. able fashion at thetop of the post 9, but I illustrate the post as having a tapered top portion 36, which receives the correspondingly formed support 3'! to which the ball 3| and the gear 33 are secured. The key 38 prevents the socket member or support 31 from turning on the central shaft 9.

It may be mentioned that the counterbalance or counterweight 21 not only counterbalances the eccentric but also counterbalances the centrifugal iorce of the entire bowl structure.

Leakage is prevented by oil wipers 40 and M. These may be kept in position by rings 42 and d3 of rubber or a suitable rubber substitute, to allow the oil wiper rings a measure of flexibility, as the eccentric 20 will not rotate exactly truly on its center, due to the large bearing clearance necessary for the proper lubrication of the machine.

The bowl structure includes the above mentioned central sleeve 35, from which outwardly extends a bottom web or plate 56 with its beveled portion 51, and any suitable circumferential flange or thickening 52. Any suitable bearing member or sleeve 53 may be interposed between the exterior of the eccentric 20 and the inner surface of the surrounding sleeve 35.

The socket 32 is supported in a sleeve or circumferential member 54 which may be secured to the plate 50 by any suitable means, such as bolts 55. It is provided with an inwardly extending upper circumferential flange 56 which rests upon the circumferential enlargement 51 of the socket 32. A cover member or feed plate 58 is held in position, for example, by bolts 59, and has a flange 66 in telescopic relationship with the exterior of the sleeve 54. It may be urged downwardly against rings of rubber or a suitable rubber substitute, indicated at 6 I.

Resting upon the conic portion i of the plate 50, and surrounding the sleeve 54, and abutting against the lower edge of the rubber ring BI, is the wear-taking liner or bottom member III which may be of manganese, steel or other suitable material. Surrounding it is the side member 1 l, the two together defining a work chamber in which material is reduced. The liners are held securely to the bowl base or bowl plate, for example, by bolts 12. A discharge aperture is defined between the outer edge of the liner in and the lower edge Y ofthe liner. The size of this discharge aperture may be controlled by washers 13. A circular collecting trough l4 surrounds the discharge outlet Z, and may be suitably bolted or otherwise secured to the bowl base or plate portion 5|, as at l5. 16 indicates any suitable discharge spout from which the particles which escape through the gap Z into the trough I4 may be delivered for storage or removal.

Any suitable oil pump, not herein shown, may be provided. 80 indicates the Oil delivery pipe, extending from a suitable oil supply. It delivers oil to the space 8! within the'base. Oil travels upwardly through the duct 82 to the axial duct 83 of the fixed shaft 3. A side duct 84 extends to a. circumferential oil channel 85 in the bearing member 2|. A side duct 86 extends to the space above the top of the eccentric. Thus the inner eccentric and the outer eccentric are both lubricated. The duct 83 further supplies lubricant to the space 81 within the ball 3|. 88 is an' 1 provide a cover in the form of a hopper BI.

the lower inner edge of which defines a discharge inlet 92 about the rubber ring 6|. If desired, a removable wear-taking member or ring 93 may be employed, seating on a bottom flange 34. 35 is a circumferential cover portion externally telescoping with the surface of the bowl member or liner H. 96 is an upper liner of yielding and wear-taking material, such as rubber or a rubber substitute.

Referring to Figures 4 and 5, 2M illustrates any suitable base or bed upon which th mill may be mounted. I illustrate a supporting frame or base including generally parallel vertically webbed I-beams 202, 203 which may be mounted on the base 20! of masonry, concrete or the like. Spanning the side frame members 2B2, 203 is a bridge member having side flanges 206, 205 and end members or angles 205, 207 the lower faces of which rest upon the top flanges of the side frame members 202, 203 respectively. The bridge member includes bottom plate portions 208, 239 which are formed to permit the movement of the impacting structure below described, being upwardly ofiset as at 2m, 2! I. 212 generally indicates a gear box mounted on the bridge, which is apertured as at 2H3 to permit the downward extension of the sleeve 2% which forms part of or which is fixed in relation to the gear box 2i2. 2l5 is a drive shaft with any suitable driving pulley 2l6 about which pass driving belts 2H. The shaft 2G5 is mounted in any suitable bearings ZiB, H9. The bearings 2i8 are located in a bearing member or bracket secured to the end portion 236 of the bridge. The bearings 219 are associated with an suitable sleeve 220 secured in relation to the gear box 2i 2. 22l is any suitable surrounding sleeve adapted to prevent penetratlon of dirt. Any suitable oil seal 222 may be employed at the outer end of the sleeve 22!, and exteriorly to the bearings H8. 223 is a cover plate for the gear box 212 but also serves to support the lower race 224 of the roller bearings 225, which'bearings support an upper race 226. Mounted on the upper race is an enlargement or head 221 of the shaft 228. Connected to the shaft 228 is a gear 229 driven by the pinion 230- on the shaft 2l5. The shaft at its-lower end is centered by any suitable bearings, such as needle bearings 23l. 232 is any suitable pump driven by the shaft 228 and is connected with an oil circuit which will later be described. The head 22'! is provided with a lateral extension 235 which, with the rest of the head, supports the lower race 236 of the rollers 231. The upper race '238 receives a centering and supporting member 239, the purpose of which will later appear. It will be understood that the axis of the member 239 is inclined in relation to the shaft 228, the two axes intersecting at A adjacent the lower end of the shaft 228. Mounted or associated with and supported upon the member 239 is "ahead 240, the opposite lower edges of which are bolted or otherwise secured to a pair of webs 24! which project upwardly ring 246, to'the lower face ofwhich is secured a cup 245, asbyany suitable bolts 246. Within the ring and the cup are a pair of bearing ele-'- ments 2", 248 which, between them, define a generally spherical concavebea ring surface concentric with the cen ter' A and opposed to and conforming to the convex beari'ngrinlg zsomounted at the lower end of the sleeve 2, and also concentric with the point A. 26! is any suitable flexible seal interposed between the ring 244 and an upper portion of the sleeve 2. 252 is any suitable clamping ring for its upper edge, its lower edge being clamped between the members 244 and 241. 253 is a preferable detachable bottom member for the cup 245, the purpose of which will later be described in detail. The cover 242 is provided with an outwardly extending flange or flanges 260 through which pass securing members or bolts 26! which may pass through a corresponding flange or flanges 262 of a lower member or container 263 having a bottom wall 264 with a central outlet aperture 265; Positioned within the member 263 is an impacting bowl generally indicated as 266 which has a somewhat conic, upwardly inclined inner portion 261 and an outwardly flared side wall 266. The member 266 may abut against the bottom 264 as at 269. It is centrally apertured as at 210, the central apertur having a conic or tapered or downwardly reduced bounding surface 21f adapted to receive the edges of a central plate 212 downwardly drawn byone or more locking bolts 213 which pass through the bottom 264 and may have external heads 214 and upper screwthreaded portions 215 penetrating the bottom of the plate 212 as at 216. It will be understood that the plate 212 and th locking member 213, may assist in centering and holding the member 266 in position. I find it convenient to associate with the plate 212 a discharge spout or tube 286 which extends downwardly through the aperture 265. Positioned above and continuing upwardly the side wall 268 is an upper liner 28L Its lower edge abuts against the upper edge of the wall 268. Its upper edge abuts against the bottom of the cover 242. When the bolts 26l are tightened the entire assembly is drawn together and the members 266 and 28l are firmly held.

It will be understood that in response to rotation of the shaft 2I5 the assembly B will be gyrated about the center A. In order to prevent rotation of the assembly B, I provide a flexible connection between it and one of the side frame members 203. I illustrate, for example, the rod 290 having at each end a head or enlargement 29| which receves the end of a flexible connector 292. Each connector 292 has enlarged ends 293 and may be provided with stabilizing or reinforcing fabric layers 294. A connector 292 may be secured to the inner head 29! of the member 290 by any suitable removable clamping member 295. Clamping members 296 are also provided on the web of the side frame member 203. Similarly the opposite end of the member 299 is secured to the side element 263 from which extends a flange 300 carrying locking means 30! for gripping the outer end of one of the connectors 292. The effect of the connector 299 is to permit free gyration of the assembly B without permitting any rotation.

Material to be ground may be delivered from any suitable spout not herein shown, to the feed receiving opening 310 in the top plate 242 of the assembly B. It will be understood that the finely divided or reduced material passes inwardly toward the center of the assembly B and escapes downwardly through the outlet tube 280 which may deliver it to any suitable receiving container, conveyor, or storage or conveying means. In order to lubricate the device I provide an oil filling passage 3 which may extend to any suitable reservoir of oil and which may be employed for delivering oil to the lubricating system below described. The oil is normally filled, in the gear box 2l2 to the level indicated at 3I2. It then passes downwardly about the shaft 228 through the clearance 3I3. It passes through and lubricates the needle bearings 23! and down to the bottom space 3 passing downwardly through the channel 3l5. It thus serves to lubricate the spherical bearing surfaces of the members 241, 246 and 260. It is withdrawn by the gear pump 232 and driven thereby upwardly through the central oil duct or passage 316 which extends up through the center of the shaft 228. It passes thence through the head 221, which is welded to the shaft 228 and passes into the interior of the cap 239 as at 311. It lubricates the upper roller bearings 231 and passes downwardly through the passage 3! to lubricate the lower bearings 226. It then passes downwardly through the opening 3| 9 to the gears 223 and 230. A certain amount of oil also passes outwardly along the shaft 2l6 to lubricate the bearings 2 l8 and 2 l9. Any suitable compression seal 32!] may be employed to prevent leakage about the member 220 at its junction with the gear box 2 I 2.

The oil seal 222 of leather or the like being located exterior to the bearings 2I8, prevents escape of oil at that point. 322 is any suitable top oil seal about the cap 32I which is secured to the head 221 and extends about the member or cup 239. A sealing connection is also indicated at 323 exterior to the lower roller bearings 225.

It will be realized that whereas I have described and shown two practical and operative and substantially different mechanisms for employing or carrying. out my grinding method, a still further substantial range of mechanisms may be employed. It is possible widely to vary details of the mechanisms employed, while still carrying out the characteristic steps and employing the characteristic features of my method. For example, the slope of the inner portion of the bowl may be widely varied in order to control the relative position or contour of the material inside the bowl. The length of stroke and speed of stroke may be varied within a substantial range.

The use and operation of the invention are as follows:

Referring to both forms of the device, I show a grinding mechanism for reducing relatively small particles to still smaller size, in which I gyrate or toss a bowl or plate about a predetermined center located somewhat above the back of the plate. The elevation of the center, such as is shown at X in Figure 1, may be governed by a desired outward component of projection of the material upwardly projected when any given part of the bowl bottom or plate moves upwardly.

The bowl or plate, such as 10 or 261, is gyrated at substantial speed. The side and top portions of the mill deflne a chamber in which the particles are ground. In both forms of the device, the material to be ground is fed generally centrally into the grinding space or chamber. It will be understood, however, that I do not wish to be limited to any specific inlet location or arrangement. In the form of Figures 1 to 3, I discharge the reduced material outwardly, through the space Y or Z. In the form of v Figures 4, 5, I discharge the material centrally through the duct 286.

In connection with the form of Figures 1 to 3, I illustrate a structure adaptable for the removal of under-sized bodies. In the course of the grinding operation, some of the balls become reduced to substantially less than optimum size. I therefore provide channels 100, which constitute, in effect, local widening of the zone Y, through which under-sized balls IUI can escape. There are only a few of these openings, but they are effective to permit the escape of under-sized balls before they reach a diameter such as to risk wedging of such balls in the space Z. In other words, before a ball can get so small as to be wedgable in the space Z, it will pass out through one of the slots I00. However, the slots I are neither so wide nor so numerous as to result in an undesirable general widening of the escape gap Z.

In any event, whatever specific type or form of impacting members may be employed, or whatever feeding or discharge means or arrangement 1 may employ, I build-up a mixed mass in the impacting chamber, and subject the mass to a series of impacts resulting from the rapid gyration of the bowl or plate structure. In effect, a wave of elevation moves circumferentially about the impact chamber. The result of the wave of elevation is to project bodily upwardly that part of the mass beneath which the wave of elevation passes. The resulting length of stroke and speed of stroke of the ring or bowl against the mass are such that the balls or impacting members, such as NH, and the particles undergoing reduction, or grinding, are thrust upwardly from and by the bowl far enough and high enough so that when the balls or impacting members and the particles fall, they do not contact the surface of the bowl until the underlying part of the bowl has again been down to its lowest level and is advancing upwardly. The result is that when any particular part of the bowl is traveling upwardly, it will contact the downwardly falling particles and impacting members after they have received a substantial degree of gravital acceleration. The result is an impact of maximum force and effect. It should be noted that the mass of any individual impact element or ball is so small, in relation to the mass of the bowl, that the factor of inertia of the individual ball or impact element is unimportant. The result is the subjection oi' the particlesto violent impact when the balls or impact elements contact and are contacted by the bowl. The force of this impact is distributed through the mass of impact elements and material undergoing grinding so that the faces of the individual balls or elements serve as impacting faces which exercise a sharp impact against material trapped between adjacent balls and in the line of the blow delivered by the bowl. In

effect, I obtain a multiple impact, because the mass of balls and particles separate out or scatter on the upward and downward stroke. This eilect is particularly marked if the machine is operated wet, with water supplied to the interior of the impacting chamber. The final or bottom particles are struck first and the result is an instant and complete reversal of direction of movement of the particles and impact members.

These, in turn, are carried upwardly to impact the still falling upward part of the mass. This action is entirely unlike anything found in prior art ball mills, and involves a multiplication or sequence of impacts at a single movement of the bowl. Each ball or impact member, which has just changed its direction of movement, will strike against the mass above it just as hard as did the bowl itself. This is necessarily so because it is now traveling in reverse direction at the same speed as the speed of travel of the bowl. The mass of the bowl is so great, in relation to the mass of any individual particle or impacting member, that it is not perceptibly slowed down by the successive impacts against individual impact members and it continues to multiply its breaking effect by carrying the already impacted members and particles upwardly against the still downwardly falling balls and particles. As a result, the theoretical slowing down of the bowl or plate by its impact against the particles and impacting members is unimportant. It is a simple matter so to relate the length and the speed of gyration under load as to obtain the desired impacting effect.

An advantage of my method and structure is that the subjection of the bowl to impact is always localized. For example, only about oneeighth of the bottom of the bowl will, at any instant, have material in contact with it which is being accelerated, if the general proportions of the drawings of the present application are followed.

The rest of the bowl area will not be under strain, as the material will be clear of the bottom of the bowl. In other words, no more than one eighth of the circumference of the bowl is performing work at the same time. And the zone of simultaneous strain is moving rapidly around the bowl, so that no part of the bowl'is subjected to a continuing strain due to impacting the charge. While this may not be necessary for the practice of the invention, it is helpful in connection with the design and employment of commercially acceptable machines for carrying out the invention.

I claim:

1. In a grinding mill, an impacting bowl mounted, on an inclined axis, for gyration about a central point, said bowl having a bottom with a discharge aperture located in an area remote from the central portion of the bowl, a plurality of movable impacting elements in said bowl, means for delivering material to be ground to the interior of said bowl, and means for tilting and gyrating said bowl about a central point through an excursion of sufficient vertical amplitude, speed and frequency to throw a substantial proportion of the mass of material to be ground, together with its associated impacting elements, upwardly clear of the bottom of the bowl at each upward gyration of the bowl, said bowl being adapted to receive said mass of material when it drops, the mass of the bowl being substantially greater than the mass of the material which is simultaneously received at anyone time by the 2. The structure of claim 1, characterized by and including a generally circumferential discharge aperture located generally adjacent the outer edge of the impacting plate.

3. The structure of claim 1, characterized by and including an impacting plate having the general form of a truncated cone.

4. The structure of claim 1, characterized by and including an impacting plate having the general form of a truncated cone, and having a portion of increasing angularity adjacent the upper portion of said truncated cone.

5. The structure of claim 1, characterized by and including a cover member constituting a hopper and terminating with a discharge apertur vertically aligned with a central portion of the bowl.

6. In a grinding mill, an impacting bowl mounted on an inclined axis for gyration about a center located above the bottom of the bowl, said bowl having a bottom wall and a circumferential outer wall upwardly extending above the outer edge of the bottom wall, said bottom wall and outer wall being separated by a discharge aperture, a plurality of movable impacting elements loose in said bowl, means for delivering the material to be ground to the interior of said bowl, and means for gyrating said bowl through an excursion of suflicient vertical amplitude, speed and frequency to throw a substantial proportion of the mass of material to be ground, together with impacting elements mixed therewith, upwardly clear of the bottom of the bowl, at each upward gyration of the bowl bottom, said bowl being adapted to receive said mass of material when it drops, the mass of the bowl being substantially greater than the mass of the material which is simultaneously received by the bowl at one time.

'I. The structure of claim 6, characterized by and including a circumferential discharge aperture.

8. The structure of claim 6, characterized by and including an adjustment for varying the effective cross-section of said discharge aperture.

9. In a grinding mill, a base, a mill body movably mounted on said base,.on an inclined axis, for gyration about a predetermined center, an impacting bowl removably mounted on said body, said bowl having a discharge aperture adjacent its outer edge and remote from its axis, a plurality of movabl impacting elements loose in said bowl, means for delivering the material to be ground to the interior of said bowl, means for gyrating said bowl through an excursion of sufficient vertical amplitude, speed and frequency to throw a substantial proportion of the mass of material to be ground, together with impacting elements mixed therewith, upwardly clear of the bottom of the bowl, at each upward gyration of i the bowl bottom, said bowl being adapted to receive said mass of material when it drops, the mass of the mill body and bowl being substantially greater than the mass of the material which is simultaneously received by the bowl at one time.

10. The structure of claim 9, characterized by and including a bowl having a bottom wall and a circumferential side wall, said bottom and side walls being removably and separably secured to said mill body.

11. The structure of claim 9, characterized by and including a bowl having a bottom wall and a circumferential side wall, said bottom and side walls being removably and separably secured to said mill body, the outer edge of the bottom wall and the lower edge of the side wall defining a generally circumferential discharge aperture.

12. The structure of claim 9, characterized by and including a bowl having a bottom wall and a circumferential side wall, said bottom and side walls being removably and separably secured to said mill body, the outer edge of the bottom wall and the lower edge of the side wall defining a generally circumferential discharge aperture, and an adjustment for adjusting the distance separating said opposed edges, and for thereby adjusting the effective cross-sectional area of the discharge aperture.

13. In a fine grinding mill, a bowl mounted 10 on an inclined axis for gyration about a predetermined center, said bowl having an outlet located adjacent its outer edge, and having a bottom portion and a circumferential side portion upwardly extending above the periphery of the bottom portion, a plurality of impacting members located within and movable in relation to the bowl, and movable in relation to each other, means for delivering material to be ground to the interior of the bowl and means for mating said bowl about its center of gyration through an excursion of suflicient vertical amplitude, and at sufllcient linear speed, to carry the bowl and the mixed mass of impacting members and particles undergoing grinding rapidly upwardly on the up stroke, and to withdraw the bowl from supporting relationship with a substantial proportion of said mass, on the down stroke, whereby the mass is permitted to fall freely under gravity until again caught by the bowl on the up stroke, the mass of the bowl being substantially greater than the mass of the material which is simultaneously contacted at any one time by the bowl.

14. In a grinding mill, a laterally extending impact plate mounted on an inclined axis, housing members associated with the plate for defining an impacting chamber, a plurality of impasting elements movable in said chamber, means for delivering the material to be ground to the interior of the chamber, means for gyrating said plate and for thereby moving it upwardly and downwardly with a circumferential wave motion, through an excursion of suiilcient vertical amplitude, speed and frequency to throw a substantial proportion of the material to be ground, together with the associated impacting elements, bodily upwardly from the upper surface of the plate, at an area which progressively moves about the plate, the mass of the plate being substantially greater than the mass of the material, which is simultaneously received by the plate at any one time, said plate having a generally circumferential discharge outlet positioned adjacent the outer edge of the plate.

15. The structure of claim 14, characterized by and including a downwardly and inwardly inclined hopper, constituting a cover for said impacting chamber and having a discharge aperture adjacent the central portion of said chamber.

16. The structure of claim 14, characterized by and including parts defining a circumferential discharge outlet having a plurality of enlarged portions adapted to permit the escape of under-sized media.

1'7. The method of reducing particles which includes maintaining a generally cylindrically bounded grinding zone having a generally upright axis, maintaining in said zone a mixed mass comprising particles of material to be reduced, and separate loose impacting elements sufficiently massive to exert a crushing action on said particles by impact therewith, feeding additional particles, for reduction, into a receiving space adjacent the center of said zone, moving said particles generally outwardly, during their reduction, removing the reduced particles from the periphery of the mass, adjacent the outer circumference of the grinding zone, and meanwhile subjecting the mixed mass, from below, to crushing impacts effective bodily to move upwardly the impacted mixed mass of particles and impacting elements, causing the mass to fall REFERENCES CITED The following references are of record in the file of this patent:

Number Number Name Date Voee Dec. 4, 1855 Hathaway June 19, 1900 Pendleton Aug. 5, 1902 Daniel at al. Jan. 17, 1939 Symons Jan. 8. 1943 FOREIGN PATENTS Country Date Great Britain 01 1895 Great Britain Dec. 3, 1931 Great Britain May 27, 1938 Germany June 6, 1913 Germany Jan. 11, 1921 

